Traditional offset printing does not allow for variable data printing. The inking subsystem used applies ink over a static plate image. Typically, ink is depleted from an inker form roll as the ink is transferred onto the imaging plate, the ink form roller being the last roller that is in direct contact with the imaging plate. Different regions of the imaging plate may need more or less ink depending upon which regions are oleophilic foreground areas and which regions are oleophobic background image areas. Traditional offset ink delivery systems adjust ink flow to different regions of the plate using manually adjusted keys which change the ink feed rate in order to guarantee enough ink will flow in solid imaging regions but prevent too much ink from flowing to areas covered by fine lines or half tones.
Recently, keyless inker systems have been introduced which meter ink appropriately without the need for inker keys. Exemplary keyless inker systems include those sold by Koenig & Bauer AB group (KBA) located in Germany. Such keyless systems use a metered anilox roller to pull fresh ink uniformly out of an ink tray and deliver the ink directly to a rubber form roll which then transfers the ink to the an imaging plate. Such systems provide for more consistent ink flow regardless of whether a solid or fine artwork is being printed. However, the ink thickness that remaining on the form roller after being partially transferred to the static image on an offset plate is not uniform. This is because ink splits onto the imaging plate in imaging areas but is fully rejected in non-imaging areas by the dampening fluid. Thus the remaining non-uniform ink thickness on the form roller has a thickness pattern which reflects the image pattern printed onto the static plate. Thus not all areas on the form roll are covered with the same thickness of ink after transfer of ink onto the imaging plate and when new ink is transferred onto the form roller some of the old thickness pattern partially remains. To minimize these effects, keyless inking systems include a form roll that has a soft or conformable surface, an anilox metering roll, and imaging plate that are all substantially equal in diameter. Further, since these rollers are all of equal diameter, related art keyless inking systems typically have large diameter anilox meter rollers and form rollers since the image plate is large in area, for example a B2-size sheet format.
The reason for equal diameters of the rollers is so that history effects add “in-phase” with the image on the plate. The form roller then builds up a reproducible ink layer thickness “in phase” with the static offset plate image.
However, when changing print jobs from one static imaging plate to the next, there is thus some ghosting and some make ready printing necessary to erase the history of the prior ink film thickness distribution on the form roller. This make ready allows time for the new equilibrium ink film thickness to build up “in-phase” with a new plate image over time. Thus related art keyless systems still suffer from some ghosting and necessary make ready between print jobs.
For a variable data lithographic printing inker system, the ink film thickness must always be the same regardless of the imaging history because a new image is introduced on each pass of the printing process. This is because a new pattern of dampening solution is formed by laser evaporation on each pass of the imaging cylinder containing a reimageable print surface. In addition, variable data lithography is different from static offset lithography because the ink is transferred directly to an elastomeric conformable blanket that holds the latent image in the dampening fluid after it has been laser patterned in contrast to traditional offset which holds a static fluid pattern over a hard metal offset plate surface. Thus a new inker system must be designed to be compatible with the new requirements of a variable data lithography print system.